The present invention relates to a semiconductor device having a gate electrode composed of a multilayer film comprising a lower-layer titanium nitride film and an upper-layer tungsten film and a method for fabricating the same.
With the recent advancement of technology for providing a higher-speed semiconductor integrated circuit with a higher integration density, a MOSFET has been miniaturized increasingly.
However, if a gate insulating film for a conventional gate electrode composed of a polycrystalline silicon film is thinned more and more with the miniaturization of the MOSFET, the driving force of the MOSFET lowers markedly due to the depletion of the gate electrode.
To circumvent the problem, attention has been focused recently on a metal gate process which uses, as a gate electrode, a metal film free from the depletion of the gate electrode. The metal gate process is effective in reducing a delay in the gate electrode since a materiel composing the gate electrode of the metal gate has a low resistance value.
As one of structures of the gate electrode, there has been used a multilayer structure comprised of a lower-layer titanium nitride film (TiN) film having a thickness of about 10 to 20 nm and an upper-layer tungsten (W) film having a thickness of about 50 to 100 nm.
As conventional methods for forming a multilayer film comprising a lower-layer titanium nitride film and an upper-layer tungsten film, there have been known a first method wherein a tungsten film is deposited by CVD using WF6 gas on a titanium nitride film deposited by sputtering, a second method wherein a tungsten film is deposited by sputtering on a titanium nitride film also deposited by sputtering, and a third method wherein a tungsten film is deposited by CVD using WF6 gas on a titanium nitride film deposited by chemical vapor deposition (CVD).
In accordance with the first or second method wherein the titanium nitride film is deposited by sputtering, however, an insulating film formed under the titanium nitride film to serve as a gate insulating film is physically damaged by sputtering particles so that the reliability of the gate insulating film is lowered.
In accordance with the third method wherein the tungsten film is deposited by CVD using WF6 gas on the titanium nitride film deposited by CVD, the problem that the gate insulating film is physically damaged by the sputtering particles can be circumvented but the problem is encountered that the reliability of the gate insulating film is lowered by fluorine contained in the tungsten film. According to the report made by H. Yang (IEDM Tech Dig. (1997) pp. 459-462), the problem is encountered if a tungsten film is deposited by CVD using WF6 that a large amount of fluorine remains in the tungsten film and fluorine contained in the tungsten film penetrates the titanium nitride film to be diffused into the gate insulating film in a heat treatment process performed after the deposition of the tungsten film, which lowers the reliability of the gate insulating film.
In view of the foregoing, it is therefore an object of the present invention to provide a method for forming a gate electrode composed of a multilayer film comprising a lower-layer titanium nitride film and an upper layer tungsten film without lowering the reliability of a gate insulating film.
To attain the object, a method for fabricating a semiconductor device according to the present invention comprises the steps of: forming a first insulating film serving as a gate insulating film on a semiconductor substrate; depositing a titanium nitride film by chemical vapor deposition on the first insulating film; depositing a tungsten film by sputtering on the titanium nitride film; and patterning a multilayer film comprising the tungsten film and the titanium nitride film to form a gate electrode composed of the multilayer film.
In accordance with the method for fabricating a semiconductor device according to the present invention, the titanium nitride film is deposited by CVD on the first insulating film as the gate insulating film, so that the first insulating film is prevented from being physically damaged and the reliability of the gate insulating film composed of the first insulating film is improved.
Since the tungsten film is deposited by sputtering, there can be prevented the degradation of the gate insulating film resulting from fluorine contained in such a tungsten film as deposited by CVD using WF6 gas.
Since the tungsten film is deposited by sputtering, moreover, the degree of roughness of the surface of the tungsten film is reduced. This reduces the amount of overetching performed in forming the gate electrode by patterning the multilayer film comprising the tungsten film and the titanium nitride film and prevents penetration through the gate insulating film caused by overetching.
In accordance with the method for fabricating a semiconductor device according to the present invention, therefore, the gate electrode with a low resistance value can be formed without lowering the reliability of the gate insulating film so that a high-performance and high-reliability MOSFET is fabricated.
In the method for fabricating a semiconductor device according to the present invention, the step of depositing the titanium nitride film preferably includes the step of performing a heat treatment with respect to the titanium nitride film in an ammonia atmosphere.
The arrangement lowers the concentration of a residual impurity which is present. in the titanium nitride film and thereby prevents an increase in gate leakage current and the peeling of the titanium nitride film off the surface of the gate insulating film even if a heat treatment is performed at about 1000xc2x0 C. with respect to the titanium nitride film in the subsequent step.
In this case, the step of performing the heat treatment with respect to the titanium nitride film is preferably performed in a chamber in which the titanium nitride film has been deposited.
The arrangement lowers the concentration of the residual impurity which is present in the titanium nitride film without increasing process steps.
In the method for fabricating a semiconductor device according to the present invention, the step of depositing the titanium nitride film preferably includes the step of performing a heat treatment with respect to the titanium nitride film at a temperature not less than a temperature at which the titanium nitride film is deposited.
The arrangement lowers the concentration of a residual impurity which is present in the titanium nitride film and thereby prevents an increase in gate leakage current and the peeling of the titanium nitride film off the surface of the gate insulating film even if a heat treatment is performed at about 1000xc2x0 C. with respect to the titanium nitride film in the subsequent step.
In this case, the heat treatment is preferably performed in an ammonia atmosphere.
The arrangement further lowers the concentration of the residual impurity which is present in the titanium nitride film.
In the method for fabricating a semiconductor device according to the present invention, the step of depositing the titanium nitride film is preferably performed at a temperature not less than 600xc2x0 C.
The arrangement lowers the concentration of the residual impurity which is present in the titanium nitride film and thereby prevents an increase in gate leakage current and the peeling of the titanium nitride film off the surface of the gate insulating film even if a heat treatment is performed at about 1000xc2x0 C. with respect to the titanium nitride film in the subsequent step.
Preferably, the method for fabricating a semiconductor device according to the present invention further comprises, between the step of depositing the titanium nitride film and the step of depositing the tungsten film, the step of: forming a second insulating film on the titanium nitride film and patterning the second insulating film to form a capacitor insulating film, wherein the step of patterning the multilayer film to form the gate electrode includes the step of: forming an upper electrode of a capacitor composed of the tungsten film and a lower electrode of the capacitor composed of the titanium nitride film.
The arrangement allows formation of the semiconductor device comprising the capacitor with reduced variations in the characteristics of the electrode and reduced degradation of RF properties without increasing the number of process steps.
Preferably, the method for fabricating a semiconductor device according to the present invention further comprises, between the step of depositing the titanium nitride film and the step of depositing the tungsten film, the step of: forming a second insulating film on the titanium nitride film and patterning the second insulating film to form a hard mask composed of the second insulating film above the isolation region, wherein the step of patterning the multilayer film to form the gate electrode includes the step of: patterning the titanium nitride film by using the hard mask to form a resistor composed of the titanium nitride film.
The arrangement allows formation of the semiconductor device comprising the resistor with a high sheet resistance value without increasing the number of process steps.
In this case, it is preferred that a thickness of the second insulating film is approximately equal to or less than a height of a portion of the isolation region protruding from the semiconductor substrate.
In the arrangement, the thickness of the tungsten film remaining on each of the side surfaces of the hard mask overlying the titanium nitride film is approximately equal to or less than the thickness of the tungsten film remaining on the portion of the titanium nitride film overlying the stepped portion of the isolation region. Accordingly, the tungsten film remaining on each of the side surfaces of the hard mask overlying the titanium nitride film is removed by overetching for removing the tungsten film remaining on the portion of the titanium nitride film overlying the stepped portion of the isolation region, so that variations in the characteristics of the resistor are prevented.
A first semiconductor device according to the present invention comprises: a multilayer gate electrode comprised of a lower-layer gate electrode composed of a titanium nitride film deposited by chemical vapor deposition on a semiconductor substrate and of an upper-layer gate electrode composed of a tungsten film deposited by sputtering on the titanium nitride film; and a capacitor comprised of a lower electrode composed of the titanium nitride film, of a capacitor insulating film formed on the lower electrode, and of an upper electrode composed of the tungsten film formed on the capacitor insulating film.
The first semiconductor device according to the present invention implements a semiconductor device comprising a reliable gate electrode and a capacitor with reduced variations in the characteristics of the electrode and reduced degradation of RF characteristics.
A second semiconductor device according to the present invention comprises: a multilayer gate electrode comprised of a lower-layer gate electrode composed of a titanium nitride film deposited by chemical vapor deposition on a semiconductor substrate and of an upper-layer gate electrode composed of a tungsten film deposited by sputtering on the titanium nitride film; and a resistor composed of the titanium nitride film.
The second semiconductor device according to the present invention implements a semiconductor device comprising a reliable gate electrode and a resistor with a high sheet resistance value.